There are many previously known resonators for use in magnetic resonance (MR) systems for imaging and spectroscopy. For example, one previously known device is commonly known as a birdcage resonator. These previously known birdcage resonators typically comprise a plurality of circumferentially spaced inductive/capacitive elements connected by inductive/capacitive end ring elements, which are driven at resonance by a Larmor radio frequency useful for nuclear magnetic resonance (NMR) systems. The object to be analyzed, e.g. the brain, is positioned within the resonator during the operation of the MR system.
One disadvantage of these previously known resonators, however, is that they typically use lumped element circuits (with discrete inductors and capacitors) to achieve resonance at the selected radio frequency. Because lumped element circuits lose more energy to radiation at high frequencies where the circuit is greater than 1/10 wavelength, the lumped element resonator is less efficient for high field MR imaging applications compared to lower field strengths. Because lumped element circuits are more radiative, they are electrically less efficient and have a lower Q factor. Similarly because conventional lumped element circuits such as the birdcage are more inductive, they resonate at lower frequencies than do the less inductive transmission line (TEM) circuits.
These previously known resonators, which use lumped element circuits, suffer from several additional disadvantages. One such disadvantage is non-uniform current distributions which result in decreased homogeneity, decreased fill factor, and increased electric field losses. Especially at higher frequencies and for larger (clinical) volumes, lumped element resonators may achieve self-resonance below the desired frequency of operation as well as increased electromagnetic radiation losses.
A distributed circuit, cavity resonator for NMR systems disclosed by Vaughan in U.S. Pat. No. 5,744,957 overcomes the above-mentioned disadvantages of the previously known devices. Vaughan discloses a cavity resonator having coaxial inner and outer cylindrical walls separated by a dielectric region. Spaced conductive end walls extend across the inner and outer walls at each axial end of the coil so that the inner, outer, and end walls together form an approximate cavity. The inner, outer, and end walls can, for example, be coated with a thin foil conductive material, such as copper, silver, or gold. In doing so, the coil imitates a coaxial line segment made resonant at Larmor frequencies useful for MR, such as 64 MHz (1.5 T), 175 MHz (4.1 T) or 170 MHz (4 T).
The apparatus disclosed by Vaughan provides a cavity resonator coil overcoming the problems of conventional coils discussed above, providing for a substantial improvement in signal to noise ratio (SNR). The coil will also resonate and operate efficiently at higher Larmor frequencies for the higher static magnetic now available at 3 T and above. Because the SNR from MR samples increases with the magnet field strength, the ability of this coil to resonate and operate efficiently at higher frequencies means that it can be used at high field strengths to obtain further SNR gains. This increase in signal to noise ratio provides a substantially improved image of the object to be analyzed within the resonator during the operation of the MR system, for example.
The resonator disclosed by Vaughan has proven effective in MR systems providing increased SNR, homogeneity, transmit efficiency, fill factor, decreased electric field losses, and higher operational frequencies. However, there are still some problems associated with both the resonator disclosed by Vaughan and the previously known lumped element resonators.
An ideal clinical head coil for example would lend itself to the easiest and most comfortable subject positioning, leave the subject's face uncovered once the subject is in position, and would include these access and comfort features without compromising coil performance, safety and reliability. It is preferable for a head coil to be both comfortable and accessible for the subject and easy to use for the technician. However, it is difficult to provide comfort and accessibility for the subject and ease of use for the technician and maintain a high coil performance. The ideal coil should have a removable top as well, to allow for comfortable subject positioning in the coil. Furthermore, some commercial systems don't provide the space for a coil that slides over a subject's head. Accordingly, several commercial coil designs already incorporate this “pop top” feature. However, these commercial coils that separate into halves are not popular with some research applications such as fMR. Apparently the electrical contacts that are broken and remade to open and close the coil each time a new subject is loaded, become unstable over time due to wear and oxidation, resulting in noise “spikes” and temporal instabilities often seen in EPI images for example. These electrical contacts are required to complete the end ring current paths in birdcages and similar coil structures. While commercial coils must meet rigorous FDA safety criteria, it could be imagined that electrical contacts in a coil might possibly pose safety risks in certain situations, especially where electrolytic bodily fluids were present.
Similarly, an ideal body coil might be as small as possible and fit close enough to the human trunk for efficient transmission to and reception from a region of interest, but allow room for subject comfort and access. The present body coils are built very large to allow for access and comfort, but as a consequence are very inefficient and are poorly couplet the MR region of interest in a body. RF power amplifiers of 35 kW and more are required to compensate for the inefficiencies of a body coil used in a 3 T magnet for example. Still, these coils provide little shoulder room for the largest human subjects.
Limb coils, especially leg coils, are similarly limited by conventional practice. A leg volume coil for example must be oversized to make room for a leg with a foot to be threaded through the cylindrical structure. Or a leg coil has a removable top to provide easier access for a closer fitting, more efficient coil. This latter coil however by conventional designs requires the problematic electrical contacts described for the head coil above.
Typical existing head only MR systems are one-piece units. A significant problem with this structure is that many medical subjects do not possess the physical ability to manipulate their heads and bodies into the positions required for the MR without significant difficultly or pain. Typically, the subject must try and navigate their head into the small diameter of the head only system. This can be painful or impossible for most medical subjects who are asked to do this while lying on their back.
Because of the inherently low SNR of the MR signal, these signals must be acquired and averaged a sufficient number of times to improve the SNR to a significant value. MR data acquisition by signal averaging is highly intolerant of motion in the MR sample or subject. Accordingly, human subjects are required to remain motionless for the duration of an MR scan, sometimes lasting 30 minutes or more. Movement will result in lower resolution images and in image “ghosting”, thereby limiting the diagnostic quality of an image and often requiring a repeated imaging session. To minimize head motion for example, MR operators will often insert padding around the subject's head to provide head restraint. While this has the effect of reducing the subject's head movement, it does not eliminate all of the subject's head movement. Further, all of the padding placed around the subject's head can apply uncomfortable pressure and can intensify the subject's feelings of claustrophobia. Therefore, the purpose behind having a high performance coil with a better signal to noise ratio is defeated if the subject cannot remain still.
Another, problem associated with many MR protocols, is they can be painfully loud. Typically, subjects are given earplugs or headphones to muffle the noise (in most MR centers the subject can even bring their own cassette or CD to listen to). The acoustic noise is attributed to the electro motive forces generated by switched electrical currents in the wires of the magnet's gradient coils. Stronger magnet fields and stronger or faster gradient current switching generate greater acoustic amplitudes. While the methods mentioned above are generally effective for gradient noise reduction, coils must be built to larger and less efficient dimensions to accommodate the head restraint and hearing protection devices placed about the head.
Visual input/output is often required for a subject receiving an MR exam, for diagnostics or research. These I/O visual devices help to minimize claustrophobia, provide visual cues, and relay information from the MR operator. Visual I/O devices are typically mirrors, prisms, or active displays located above the subject's eyes. A problem with existing systems of this sort is that 1) they are often fixed in position which requires that a subject be adjusted to the device, and 2) they typically protrude above the head coil so as to preclude their use in close fitting head only MR systems, and in head gradient inserts used in whole body MR systems.
It has been shown that back planes on RF coils can function as an RF mirror to extend the uniformity of the coil's transverse RF magnetic field along the rotational or “z” axis of the coil. A back plane, also known as an end cap, can be used in a coil to make a shorter, and therefore more ergonomic, better shielded, and more electrically efficient coil for a desired field of view. Conventional cylindrical birdcage head coil, as mentioned above, typically do not have a back plane. The lack of a back plane together with the inherently shorter axial field of a birdcage require the birdcage head coil to be longer typically covering the subject's mouth and chin. This increased length of the birdcage has many problems. It creates a head coil, which can increase feelings of claustrophobia for the subject. Once inside of the head coil the subject's mouth is located immediately in front of the inside coil wall. A subjects breath pushed back into their face by the inside coil wall creates a very uncomfortable/claustrophobic feeling for the subject. This is an undesirable result since the MR exam may take 20 to 90 minutes. Additionally, general medical access and vocal communications are impeded with the coil extending over the subject's mouth and chin. Further, if the subject has a large head, nose, and/or chin it becomes increasingly difficult to fit the subject's head inside of the coil. Another additional disadvantage for coils not having end caps is additional electromagnetic energy is lost from the top of the coil and thus the coil is less efficient at high frequencies.
While a back wall in a head coil is more desirable for coil performance and ergonomics in relation to the subject's mouth and chin (i.e., with a back wall the head coil body can be shorter and thus the head coil does not have to extend over the chin), a back wall is undesirable for a couple of reasons. First, a back plane closes off one end of the coil giving the appearance of putting ones head into a bucket rather than an open cylinder. This can increase a feeling of claustrophobia for the subject. Second, the back plane limits access to the subject from the back of the magnet. In coils without back planes, leads for physiological monitors, anesthesia and/or respiratory hoses, EEG leads, communications lines, etc., can be passed. In these systems visual signal projection is often performed from the rear of a magnet and through the back of a coil to mirror or prism systems mounted above the subjects' eyes. Therefore, presently, head coil manufacturers must choose between the benefits of having a coil back plane or end cap or the benefits associated with access to the subject provided by head coils with no end cap.
As stated above, a problem associated with head coils is the amount of space provided inside of the coil. RF coils transmit MR stimulus to the subject and receive signals back most efficiently when the coil is as close as possible to the subject. Therefore, for RF coil performance considerations, space inside a coil should be only enough for subject comfort and for the inclusion of devices useful for safety, head stability, and communication with the subject. As stated above, normally a subject must wear earphones or plugs for hearing protection and have separate pads inserted around the head to hold the head motionless for the MR exam. Further, there is typically some sort of visual and/or audio communications device adjacent to the head so that the MR operator can communicate with the subject. However, the padding, hearing protection and communication equipment can not only make the MR experience uncomfortable for the subject but this equipment also occupies limited space within the head coil.
All of these problems listed above, individually and collectively, degrade the overall quality of NMR images and spectra, add to the discomfort of the subject, and limit subject access for the physician or researcher. Therefore, what is clearly needed is a high performance apparatus, which provides for increased signal to noise ratios and improved MR image quality, while overcoming the problems discussed above.